Locking devices are needed in various settings to prevent, and selectively allow, longitudinal translation of an object, such as a rod, relative to another body. Cooper Cameron Corporation provides a mechanical device on a subsea collet connector that is used to secure a blowout preventer stack to a wellhead to prevent the inadvertent unlocking of the collet connector. The Cooper Cameron Corporation lock is shown generally at 10 in FIG. 1. A pair of opposed jaws 12 and 14 provide an opening within which a rod (not shown) is located. The jaws 12 and 14 are fixed to a frame 16 with shoulder screws that restrict axial movement but allow limited lateral movement of the jaws relative to the frame. The frame 16 is appropriately mounted on the collet connector. A bracket 18 receives two guide bolts 20 in appropriate holes. The guide bolts 20 also pass through appropriate holes in the jaws 12 and 14. The jaws 12 and 14 are movable along the guide bolts 20 within the restrictions of the shoulder screws used to mount the jaws on the frame 16. Nuts at the ends of the guide bolts 20 keep the guide bolts confined within the holes in the bracket 18 and the jaws 12 and 14. Spring washers 22 are positioned along the guide bolts 20 between the bracket 18 and the jaw 12, urging the jaw 12 toward the jaw 14. Thus, the spring washers 22 tend to force the two jaws 12 and 14 together around the rod that passes between them. The surface of each of the two jaws 12 and 14 that contacts the rod is curved, and lined with grooves and teeth 24. The teeth 24 engage the rod by being received within like grooves defining teeth on the rod, to hold the rod against longitudinal movement relative to the frame 16. Only by application of sufficient longitudinal force on the rod can the teeth and grooves of the rod and the jaws 12 and 14 interact to force the jaw 12 to be pushed against the spring washers 22, moving away from the jaw 14 and allowing the teeth of the rod and jaws to disengage to allow the rod to move. Friction and mechanical spring forces thus provide the resistance to hold the rod against movement relative to the frame.
A pin 26 is threaded through a hole in the bracket 18 and can be advanced to engage the jaw 12 and so limit the movement of the jaw 12 and the bracket toward each other, while the guide bolts 20 limit movement of the jaw 14 away from the bracket. The pin 26 can thus lock the jaws 12 and 14 against movement away from each other with a rod engaged by the jaw teeth 24, and so lock the rod against longitudinal movement relative to the frame 16.
The mechanical lock 10 can be too large for some applications. Further, the bolts 20 and the spring washers 22 have a tendency to corrode in the subsea environment. Spring washers require exotic materials in order to fit a properly designed washer inside limited space and generate the forces required. This makes the spring washers expensive.
Another locking device provided by Cooper Cameron Corporation employs a housing containing a stack of flat washers that fit loosely around a rod that extends along a passage through the housing. A coil spring pushes longitudinally on one side of the stack of washers to tilt all of the washers relative to the rod. The slanted washers thus engage and bind the rod by friction against longitudinal movement through the housing. A pin is provided whereby force may be applied to push the washers against the spring to lower the tilt of the washers and release the rod for longitudinal movement relative to the washers and the housing.
Another type of locking device is that sold under the registered trademark BEAR-LOC®. This lock includes a metal sleeve, and is based on the principle of elastic expansion of metal under pressure. The sleeve surrounds a rod and grabs the rod with a consistent amount of force, and is released when hydraulic pressure is applied to the device. Aside from the tendency of this style of lock to be expensive, the sleeve must be long to provide enough surface area to generate the friction required for significant resistance. Further, since the device works off of friction between the sleeve and a polished rod, there is a tendency for the resistance to decrease with number of cycles as the sleeve wears.
It is advantageous and desirable to provide an improved locking, or braking, device for retarding translation of a rod, for example, relative to another body. It is an object of the present invention to provide such a device that can be made in a wide range of sizes, and that requires no highly technical equipment to operate. The present invention meets these needs.